This invention relates to resinous compositions of matter, and more specifically to resinous polycarbonate blends having improved weatherability.
Polycarbonates are a widely employed class of polymers, in part because of their excellent physical properties including high impact strength. However, their long term color instability is a problem. It causes yellowing of the polymer, detracting from its transparency and attractiveness. Loss of gloss can also be an undesirable long term phenomenon in polycarbonates and, especially, in blends of polycarbonates with polyesters such as poly(alkylene terephthalates).
Yellowing of polycarbonates is caused largely by the action of ultraviolet radiation, which is why such yellowing is frequently designated "photoyellowing". Numerous means for suppressing photoyellowing have been employed and proposed. Many of these involve incorporation in the polycarbonate of ultraviolet absorbing compounds (UVA's). For the most part, UVA's are low molecular weight compounds and they must be employed at relatively low levels, typically up to 1% by weight, to avoid degradation of the physical properties of the polymer such as impact strength and high temperature properties as reflected in heat distortion temperature.
Numerous other polymers have been blended with polycarbonates for various purposes. For example, U.S. Pat. No. 3,792,115 describes blends of polycarbonates with polyarylates derived from a varied and large selection of dihydroxy compounds such as hydroquinone, resorcinol and 2,2-bis(4-hydroxyphenyl)propane ("bisphenol A") and dicarboxylic acids such as terephthalic and isophthalic acids, said blends being characterized by improved physical properties such as impact strength and heat deformation resistance.
Other polymer blends are also described as having improved properties. U.S. Pat. No. 4,246,381, for example, describes blends of a polyarylate, which may be a bisphenol A-hydroquinone terephthalate-isophthalate, with a copolyester derived from a cyclohexanedimethanol, an alkylene glycol and an aromatic dicarboxylic acid, said blends having improved mechanical properties after exposure to ultraviolet radiation. U.S. Pat. No. 5,552,463 and Japanese Kokai 1/201,326 describe blends of poly(ethylene terephthalate) with a resorcinol isophthalate, terephthalate and/or aliphatic dicarboxylate, characterized by improved high temperature and gas barrier properties. None of these publications, however, offer any guidance on improvement of polycarbonate resistance to degradation by ultraviolet radiation and loss of gloss, hereinafter sometimes collectively designated "weatherability".
Copending, commonly owned application Ser. No. 09/030,076, now U.S. Pat. No. 5,916,997, describes weatherable "soft block" copolyesters comprising resorcinol iso/terephthalate units in combination with ester units derived from an aliphatic or alicyclic dihydroxy compound or dicarboxylic acid. There is no suggestion, however, of the use of such copolyesters in blends with polycarbonates.